Liquid foods, particularly fruit and vegetable juices, fruit extracts, herb extracts, and the like, are examples of aqueous solutions that have temperature-sensitive properties, such as color or aroma, or that contain temperature-sensitive substances such as, vitamins or other active substances. Such aqueous solutions usually are obtained with very low concentrations of solids in the range from about 5 to 15 percent by weight of dry substance, based on the total weight of the solution. In general, it is desired to increase the solids content of these solutions by the removal of water, the most varied methods of concentration being used for this purpose.
The evaporation method frequently used for concentrating has the major disadvantage of exposing the solution, and thus also the substances dissolved in it, to elevated temperatures, which usually results in a concentrated product of low grade. Elevated temperatures always cause damage to the temperature-sensitive substances and properties, which results in a loss of vitamins, denaturation, or changes in the product determinable by the Maillard reaction, as well as loss of volatile components, such as aromas, that are removed from the concentrate together with the separated solvent, water.
In addition to extraction processes, which can be used in a very small number of exceptional cases, ultrafiltration or reverse osmosis offers the possibility of concentration of such aqueous solutions without thermal stress on the components. However, the final concentrations achievable with these two processes are limited because of high osmotic pressure, there being from 25 to 30 percent by weight of dry substance in the concentrate.
In contrast, freeze-concentrating of aqueous solutions having temperature-sensitive components proved to be a practicable method with which maximum solids concentrations of from about 40 to 42 percent by weight of dry substance can generally be obtained, depending on the characteristics of the product. Freeze-concentrating also represents a process for further, careful concentration of such solutions since the water is not separated via the vapor phase but via the crystalline phase in the form of ice. The low processing temperatures used for this purpose prevent a loss of components as well as their damage by chemical reactions, so that the dissolved, high-quality substances remain in the concentrate in unchanged amounts and composition. The separation of the ice from the concentrate is achieved with purely mechanical means such as centrifuging, filtering, or screening or also with the aid of washing columns. At present, the freeze-concentrating method is used variously to obtain high-quality concentrates, for example, in the food processing industry for the preparation of tea and coffee extracts.
While freeze-concentrating does have the advantages discussed above, it also has certain noteworthy disadvantages. Such disadvantages include, for example, higher costs, as compared to the costs of steam concentration processes, as well as the comparatively low final concentrations of dry substance in the concentrate that can be achieved. In addition, product losses become increasingly noticeable with a rising content of solids dissolved in the concentrate, which are caused by the adhering of concentrate and pulp to the separated ice crystals. A reduction of the product losses caused in this way can be achieved only by longer washing times and thus a considerably lower output of the entire process.
Thus, there has been a need to develop a process for the gentle concentrating of aqueous solutions having temperature-sensitive components, which process, on the one hand, includes a freeze-concentrating process but, on the other hand, avoids the above-mentioned disadvantages of this method. There is a particular need to obtain a concentrate with a higher solids content.